Thin film deposition methods enable the fabrication of a wide range of useful devices and device components. Chemical and physical thin film deposition techniques, such as ion beam sputtering, plasma enhanced chemical thin film deposition, electron beam evaporation and thermal evaporation, for example, are particularly useful methods for fabricating complex structures comprising thin films of a variety of materials, including dielectric materials, semiconductors, and conducting materials. Structures have been fabricated using thin film deposition methods having selected thicknesses ranging from 10 s of nanometers to 10 s of centimeters. The applicability of thin film deposition techniques to a wide variety of deposition materials, substrates and processing conditions has lead to widespread adoption of these techniques in a number of important fields including semiconductors, microelectronics, nanotechnology, lithography and thin film optical coatings.
In thin film deposition techniques, a substrate to be coated is placed in contact with a precursor having a selected composition, such as gas phase molecules, ions, atoms and clusters thereof. Condensation of the precursor itself on a receiving surface of the substrate results in formation of a thin film layer in physical thin film deposition methods, such as ion beam sputtering and evaporative deposition techniques. Alternatively, in chemical thin film deposition methods, a substrate is exposed to a precursor, which reacts and/or decomposes on the receiving surface resulting in formation of a thin film layer having a desired chemical composition. Deposition typically occurs under reduced pressure conditions provided by a vacuum chamber for both physical thin film deposition and chemical thin film deposition techniques. In many device fabrication applications, a receiving surface of a substrate is successively and independently exposed to different precursors, thereby generating complex multilayer structures comprising a plurality of distinct thin film layers having different chemical compositions and physical properties.
In thin film deposition systems, the flux of precursors to a substrate depends on a large number of variables. First, the collision rate of precursors to the substrate surface is largely determined by the concentration (or partial pressure) and translational energy of precursors in a region proximate to the substrate surface. Second, the fraction of collisions leading to condensation on or reaction with the surface depends on the composition of the precursors, composition and morphology of the substrate surface, the electric charge at the surface, ambient pressure and ambient temperature. Finally, the flux of precursor to a substrate also depends on the geometry defining the relative positions of the receiving surface of the substrate and the thin film deposition source. The amount of material deposited onto the substrate surface at a particular point and at a particular time depends on the flux of precursors to a substrate and on the amount of time this point on the substrate surface is exposed to the deposition flux. For many thin film deposition systems, a number of these variables vary from point to point across a receiving surface; therefore, the flux of precursors to the substrate surface is often spatially inhomogeneous.
In many cases, the utility of thin film deposition methods for device fabrication applications is dependent on the capability of these techniques to generate thin film layers having uniform physical thicknesses, chemical compositions and physical properties, such as refractive index, density, optical thickness and surface roughness. Many thin film deposition sources, such as sputtering sources and evaporation sources, generate a spatially inhomogeneous source distribution profile of precursors. Spatial inhomogeneity of the concentrations of the precursors in a region proximate to the substrate can lead to nonuniform fluxes of precursors to the receiving surface. This spatial variability of the flux of precursors often results in deposition of thin film coatings having thicknesses which vary significantly from point to point across the receiving surface of a substrate. Such variability in the thickness of deposited thin film layers can impede the performance of devices fabricated using thin film deposition methods. For example, spatial variability in the thickness of thin films comprising multilayer thin film optical components, such as optical interference filters and antireflection coatings, may result in devices having optical transmission and/or reflection properties that vary as a function of position of a beam on an incident surface.
A number of techniques exist to address problems associated with spatially inhomogeneous source distribution profiles of precursors generated by many thin film deposition sources. One useful method for achieving thin films having improved thickness uniformity uses substrate positioning systems, such as planetary systems, that translate and/or rotate a substrate in a flux region during the thin film deposition process. In a dual planet planetary system, for example, one or more rotating planets carry substrates and the rotating planets are translated in a circular orbit about a central rotational axis during deposition. Translation and rotation of a substrate in this system exposes different areas of its receiving surface to different precursor concentrations and energies in a given source distribution profile of precursors, thereby providing substantially similar average fluxes of precursors over a selected deposition time to all areas of a substrate surface. Use of substrate positioning systems, such as planetary systems, has been demonstrated to generate thin films having enhanced spatial uniformity with respect to the thickness, composition and morphology of deposited layers.
In addition to the need for good uniformity, the utility of thin film deposition fabrication methods in many applications is also critically dependent on the capability of these techniques to generate thin films and multilayer structures having precisely selected thicknesses and chemical compositions. For example, the optical properties of thin film optical components, such as optical interference filters and antireflection coatings, strongly depends on the physical thicknesses, refractive indices and compositions of individual layers comprising a multilayer optical device. Fabrication methods for making multilayer optical devices having stringent optical specifications, therefore, require accurate and sensitive means for evaluating deposition conditions for determining when a desired thickness of a component thin film layer is achieved and, thus, deposition of a subsequent thin film layer is to be commenced. As a result of these requirements a number of techniques have been developed over the last several decades to monitor and control physical and chemical properties of deposited thin films during thin film deposition, such as the thicknesses of deposited thin film layers and their chemical composition and optical thickness.
The average flux of precursors provided by many thin film deposition systems, such as those employing sputtering sources and evaporation sources, undergoes significant variations during deposition of a thin film. Therefore, fabrication methods for making thin film devices having stringent device tolerances require accurate means of monitoring the flux of precursor materials to a substrate in real time. In a conventional thin film deposition system, the flux of precursor materials is usually determined using a fixed position sensor, such as a quartz crystal microbalance sensor or an optical monitor. Quartz crystal monitoring is an almost universally applicable technique of determining the mass of deposited material, which may be used to determine average layer thicknesses via calibration. In contrast to crystal monitoring, optical techniques do not provide a direct measurement of physical properties (e.g. mass or physical thickness) of the deposited layers. Rather, optical monitoring techniques detect the temporal evolution of light transmitted, scattered and/or reflected from a substrate surface having deposited layers, which may be related to thin film thickness, refractive index and composition. The temporal evolution of transmitted, scattered and/or reflected light in these systems, however, also depends significantly on the structure and optical properties of underlying materials beneath the thin film layer being deposited. For example, in situations where a thin film layer is deposited on top of a pre-existing structure of uncertain physical dimensions, composition or optical properties optical monitoring does not provide useful information in all cases. Even in the case of deposition of a thin film layer on an uncoated, well defined substrate surface, layer thickness errors from optical monitoring can oscillate out of control, and optically insensitive layers can lead to layer thickness errors that become important when an entire structure is complete. Finally, optical monitoring is often susceptible to problems associated with substrate heating and etalon effects in a given optical system. Optical monitoring also fails when appropriate light sources and detection systems are not available.
Due to the drawbacks associated with optical monitoring and the near universal applicability of crystal monitoring, crystal monitoring techniques are preferred for thin film deposition in many device fabrication applications. Quartz crystal monitoring as currently practiced, however, does not provide a direct measurement of the mass of materials actually deposited on a substrate. Rather, this technique provides a measurement of the mass of material deposited on a sensing surface of the crystal sensor. As a result of this experimental limitation, conventional crystal monitoring techniques typically employ a fixed position crystal monitor at a location proximate to the substrate surface undergoing deposition. This configuration is intended to provide measurements at the sensing surface of the monitor which may be related to actual deposition conditions at the substrate. Use of a fixed position crystal monitor, therefore, typically requires an experimental determination of the ratio of the deposition rates at the sensing surface of a fixed position quartz monitor and at the receiving surface of the substrate. This ratio, commonly referred to as the parts-to-monitor ratio, is used to convert real time measurements of the mass of material deposited on the crystal to measurements of the mass of material actually deposited on the substrate.
The source distribution profile of precursors near the substrate surface provided by many thin film deposition sources is also known to undergo variation during deposition of a thin film. These variations may be significant and typically result from changes in the operating conditions of the thin film deposition source, such as variations in ion beam intensity or target erosion in sputtering sources or variations in temperature, pressure or composition of a sample undergoing evaporation in evaporation sources. As the ratio of the deposition rates at the sensing surface of a fixed position quartz monitor and at the receiving surface of the substrate is sensitive to variations in the source distribution profile of precursors, the parts-to-monitor ratio under many experimental conditions is subject to drift during deposition of a thin film layer or during deposition of subsequent thin film layers. This drift results in layer thickness errors which can significantly degrade the quality and/or performance of thin film devices and device components fabricated via thin film deposition methods.
Alternative approaches of using crystal monitoring techniques to improve the uniformity of thin films fabricated via thin film deposition have been developed in recent years. U.S. Pat. No. 4,858,556 provides a description of a method for generating thin films employing an in situ mobile source processing monitor. In the reported technique, the “amplitude and shape” of a physical thin film deposition source is characterized “just prior to substrate processing” and this information is then used to determine “the non-linear motion scenario that is required to achieve processing of specified uniformity over a specified area.” Although the methods and devices described in U.S. Pat. No. 4,858,556 are alleged to address problems arising in “processing where source distribution profiles often vary substantially from run to run,” these techniques remain susceptible to variations in the source distribution profile of precursors occurring during deposition of a thin film layer (i.e. variations during a run, as opposed to variations from run to run). In addition, these methods require derivation of complex mathematical relationships relating a measured source distribution profile of precursors to motion scenarios of substrates required to achieve good uniformity. Mobile source processing monitor configurations described in U.S. Pat. No. 4,858,556 are limited to embodiments using “a sliding contact electrical interface 152 (FIG. 7)” which makes complex monitor trajectories, such as dual rotation trajectories, impractical and/or unfeasible. Furthermore, systems provided include mobile sensors consisting of only a crystal connecting the crystal electronically to external circuitry for reading the crystal frequency via the sliding or rotating electrical contacts. It is very difficult to control the stability of the electrical sensing circuitry in this arrangement due to anticipated changes in conductivity, capacitance, resistance and impedance upon rotation or sliding motion of the electrical contacts. Therefore, this arrangement is expected to be susceptible to substantial errors in the measured layer thicknesses arising from the sliding or rotating electrical contacts. Moreover, the configurations disclosed in this reference are not amenable to high-throughput fabrication applications.
It will be appreciated from the foregoing that there is currently a need in the art for methods and devices for monitoring and controlling thin film processing via thin film deposition. Particularly, devices and methods for monitoring and controlling thin film deposition are needed that are capable of measuring and accounting for changes in the average fluxes and source distribution profiles of precursors generated by thin film deposition sources that occur during deposition of a thin film layer and subsequent thin film layers. In addition, thin film deposition methods are needed that are capable of generating thin films having spatially uniform and accurately selected thicknesses, chemical compositions and physical properties.